Optical transmission networks allow all-optical transmission between network nodes. Traffic is carried by optical wavelength channels, called lambdas, and optical switching technology, such as Wavelength Selective Switches (WSS), allow lambdas to be switched at nodes.
A control plane can be added to this kind of network to allow automated set-up of paths, tear-down of paths and traffic recovery when faults occur in the network. A possible control plane is the Generalized Multi Protocol Label Switching (GMPLS) protocol suite being developed by the Internet Engineering Task Force (IETF). The GMPLS application for optical networks is called Wavelength Switched Optical Network (WSON).
There are now a range of different transmission technologies available for connections within an optical transmission network. Connections can operate at 10 Gbit/s, 40 Gbit/s or 100 Gbit/s and there is a range of different modulation formats, such as On-Off Keying (OOK) and various phase modulation formats, which will generally be called xPSK. Connections operating at different bit-rates and modulation formats can co-exist in the same wavelength switched optical networks (WSONs). In such a multi bit rate WSON, cross-phase modulation (XPM) can be detrimental, especially when it is induced by an OOK signal on an xPSK signal at a higher bit-rate.
A current way of establishing a connection within an optical transmission network estimates a Quality of Transmission (QoT) for the proposed connection and considers a worst-case penalty for the effects of cross-phase modulation. A value of QoT that is acceptable in the worst-case scenario (i.e. when the central wavelength is occupied by a 100 Gbit/s xPSK signal and all other wavelengths by 10 Gbit/s OOK signals) assures the preservation of the lightpath when other new lightpaths are established.
A disadvantage of the current approach is that it can result in many new connections being refused because the Quality of Transmission is unacceptable under worst-case transmission conditions.